The present invention relates to the production of single-crystal epitaxial layers of semiconductors having a ternary or more multi-component homogeneous composition and/or a uniform doping level in binary or more multi-component semiconductors, and particularly to the production of epitaxial layers from the liquid phase.
Generally, two methods of growing epitaxial layers from the liquid phase have been known. One method comprises contacting a liquid solution saturated with a source material of semiconductor to be grown with a suitable substrate and then slowly cooling the temperature of the solution to grow an epitaxial layer on the substrate. For a detailed description, see H. Nelson, "Epitaxial Growth from the Liquid State and Its Application to the Fabrication of Tunnel and Laser Diodes", RCA Review 24, p. 603, 1963. Hereinafter, this method will be referred to as the "slowly-cooling liquid phase epitaxial growth method". The second method comprises producing a temperature gradient in the liquid solution so that in a high temperature region of the liquid solution a source material of the semiconductor to be grown is being supplied and in a low temperature region of the liquid solution an epitaxial layer is depositing onto the substrate contacted with the liquid solution. In the second method, the semiconductor to be grown is transferred from the high temperature region to the low temperature region by diffusion through the liquid solution according to the temperature gradient of the liquid solution. For a detailed description, see G. B. Stringfellow and P. E. Green, "Liquid Phase Epitaxial Growth of InAs.sub.1.sub.-x Sb.sub.x ", Journal of Electrochemical Society, Vol. 118, No. 5, p. 805, (1971). This method will be referred to hereinafter as the "temperature-gradient liquid phase epitaxial growth method", though it is also called "steady-state liquid phase epitaxial growth method".
According to the first method, i.e. "slowly-cooling liquid phase epitaxial growth method", it is difficult to grow epitaxial layers on many substrates successively by using a liquid solution, since concentrations of the source material and of an impurity as a dopant in the liquid solution once used for the epitaxial growth are changed.
In the epitaxial growth of ternary semiconductors, change in composition occurs in a direction perpendicular to the growth surface and consequently the epitaxial growth of ternary semiconductors of a homogeneous composition becomes difficult. For example, in the case wherein an epitaxial layer of a ternary semiconductor Ga.sub.1.sub.-x Al.sub.x As (0&lt;x&lt;1) is grown on a GaAs substrate from a Ga-Al-GaAs mixed solution, Al concentration in an initially grown part is high and the Al concentration in the growing layer reduces rapidly thereafter. Thus, it is quite difficult to continuously grow single-crystal epitaxial layers of semiconductors having a homogeneous composition and/or uniform doping level by the first method.
Therefore, the use of the second method, i.e. "temperature-gradient liquid phase epitaxial growth method" is considered in this situation. For example, according to said report of Stringfellow et al., a homogeneous epitaxial layer of InAs.sub.1.sub.-x Sb.sub.x (0 &lt;x&lt;1) of about 80 .mu.m is obtained by placing an InAs solid source in a high temperature region of an In-As-Sb ternary liquid solution and an InAs substrate in a low temperature region thereof. However, change of the liquid solution is also caused after a number of successive epitaxial growth steps.
In Official Gazette of the Japanese Patent No. 3711/1972, a method is disclosed which comprises floating a GaAs solid source on a Ga-Al-GaAs mixed solution to grow a relatively homogeneous epitaxial layer of Ga.sub.1.sub.-x Al.sub.x As (0&lt;x&lt;1) on the GaAs substrate positioned in a low temperature region. However, a change in composition of the liquid solution is also caused if one only keeps the steady state under the fixed temperature gradient condition, since Al in the solution is incorporated in a large amount in Ga.sub.1.sub.-x Al.sub.x As. It is described therein that, for avoiding the change, a slow increase of the temperature of the whole solution as the growing proceeds is required, while the temperature gradient is always produced. Thus, change in temperature of the liquid solution is necessary as the epitaxial layer grows and, therefore, a number of continuous epitaxial growth operations are also difficult to obtain by this method.